1. Field of the Invention
The present invention relates to a mobile terminal device such as a mobile phone including two casings that are connected through a connecting unit to be able to overlie one another, and, more particularly to a mobile terminal device in which a signal-line flexible board that connects circuit boards housed in the two casings, respectively, is arranged through the connecting unit.
2. Description of the Related Art
Many mobile phones include two casings that can be overlaid one another in a folding manner or the like with a hinge structure, to meet demands for miniaturization and the like. In these mobile phones, to electrically connect circuit boards that are housed in the respective casings, a method of using a signal-line flexible board having high flexibility and providing the signal-line flexible board through a connecting unit such as a hinge structure is employed (see, for example, Japanese Patent Application Laid-open No. 2004-214443 and Japanese Patent Application Laid-open No. H7-274275).
In Japanese Patent Application Laid-open Nos. 2004-214443 and H7-274275, to shield a conductive layer not to output noises generated by the conductive layer or to prevent external noises from affecting the conductive layer, a flexible board including an electromagnetic shielding layer that covers both faces of the conductive layer having a signal line is used.
In some commercially available mobile phones, as shown in FIGS. 12 and 13, one frame-grounded flexible board 702 that is frame grounded is provided outside of plural signal-line flexible boards 701 that are arranged through a connecting unit 700 with a hinge structure, to achieve frame grounding. FIG. 12 is a perspective view of a configuration of relevant parts near a connecting unit of a conventional mobile phone. FIG. 13 is a longitudinal sectional side view of a configuration example of a part of the connecting unit where flexible boards are arranged. In FIG. 12, reference numeral 703 denotes a stationary casing including a transmitter, an operation key, and the like. Reference numeral 704 denotes a movable casing including a receiver, a liquid crystal display, and the like. The movable casing 704 has a configuration that is divided into two parts of a front armored case 705 that is located on the display face side, and a back armored case 706 that is located on the back face side, for example as shown in FIG. 13.
The casing 703 includes cylindrical bearing armored tubes 709 and 710 that are protrudingly formed on both sides of an end and into which a spindle 708 of the hinge structure is fitted. The front armored case 705 of the casing 704 includes cylindrical bearing armored tubes 711 and 712 that are protrudingly formed at an end to be located inside in the width direction of the bearing armored tubes 709 and 710 and into which the spindle 708 is fitted. The front armored case 705 includes a bearing armored unit 713 that is integrally formed abreast of the bearing armored tube 711 to form an approximately semi-cylindrical shape. The back armored case 706 includes a bearing armored unit 714 that is formed in an approximately semi-cylindrical shape at a position corresponding to the bearing armored unit 713 and with the same width as that of the bearing armored unit 713. When the front armored case 705 and the back armored case 706 are combined, the bearing armored units 713 and 714 are engaged to form a bearing armored tube 715 interiorly having a cylindrical space. The armored cases 705 and 706 are formed as metallic cases of Mg, for example, to provide the strength while the bearing armored unit 714 is formed as a separate member of a resin material such as acrylonitrile butadiene styrene (ABS). The casing 703 includes a bearing armored tube 716 that is protrudingly formed at an end to be located between the bearing armored tubes 715 and 712. The bearing armored tube 716 is formed by combining two semi-cylindrical bearing armored units, like the bearing armored tube 715.
The signal-line flexible board 701 and the frame-grounded flexible board 702 are arranged through the connecting unit 700 to pass through the bearing armored tubes 715 and 716 having cylindrical spaces. The signal-line flexible board 701 and the frame-grounded flexible board 702 are curled to make one turn in the connecting unit 700, to prevent line breaking resulting from an opening or closing operation of the casings 703 and 704 and to provide satisfactory following capability. The signal-line flexible board 701 and the frame-grounded flexible board 702 are formed to have the same width as shown in FIG. 14. FIG. 14 is an enlarged plan view of the signal-line flexible board 701 and the frame-grounded flexible board 702. The frame-grounded flexible board 702 on which a frame-grounded conductor pattern 717 is entirely formed has a coverlay 718 made of a nonconductive material on the both sides, and the coverlay 718 covers edges of the conductor pattern 717, as shown in FIG. 15. FIG. 15 is a partially enlarged plan view of the frame-grounded flexible board 702.
The user grips the mobile phone with his/her hand to operate or carry around it. When static electricity resulting from electric charging or discharging of the user enters the mobile phone, the static electricity may fall in a part where flowing of static electricity through the circuitry is undesirable. Particularly, the connecting unit 700 with the hinge structure as shown in FIG. 12 has a gap 719 between the bearing armored tubes 715 and 176. Therefore, the static electricity falling in the gap 719 enters the inner space through the gap 719, and is secondarily discharged to the signal-line flexible board 701 that are arranged in the inner spaces of the bearing armored tubes 715 and 716 to flow through the signal line. Consequently, the static electricity adversely falls in an inner circuit on a circuit board in the casing 703 or 704. In the configuration in which the frame-grounded flexible board 702 is provided outside the signal-line flexible board 701 as shown in FIG. 12 and the like, the boards are curled to make one turn in the inner space. Accordingly, the static electricity falling in the gap 719 can be secondarily discharged to the signal-line flexible board 701 prior to the conductor pattern 717 of the frame-grounded flexible board 702 and flow through the signal line. Thus, countermeasures against static electricity are insufficient.
As described in Japanese Patent Application Laid-open No. 2004-214443 and Japanese Patent Application Laid-open No. H7-274275, also when a flexible board having a conductive layer both sides of which are covered by an electromagnetic shielding layer is employed, the electromagnetic shielding layer is grounded for shielding, while it is not to prevent external static electricity from affecting the signal line. Therefore, the flexible board does not fulfill a function as a lighting conductor against the static electricity, and static electricity can flow through the signal line in the conductive layer. Thus, the countermeasures against static electricity are insufficient. That is, although the flexible board includes a grounded electromagnetic shielding layer, a gap with the signal line in the conductive layer is only about several micrometers. Accordingly, when the static electricity flows through the electromagnetic shielding layer, the signal line is right near the electromagnetic shielding layer. Thus, the influence upon the signal line is not negligible, and the static electricity is highly likely to flow also through the signal line.
In a mobile terminal device including two casings that are foldably connected through a connecting unit such as a hinge structure, it is difficult to eliminate a gap between the casings at the connecting unit because of the movable structure. Therefore, it is hard to prevent generated static electricity from entering the connecting unit.
In view of this, for the mobile terminal device in which a signal-line flexible board is provided through a connecting unit such as a hinge structure, effective countermeasures against static electricity are demanded. Particularly nowadays, the mobile phones are ordinarily indispensable devices. Because it is natural to carry around the mobile phone to operate it, countermeasures against static electricity are important, not just for dry seasons when static electricity is easily produced on human bodies.